Glass-ceramic articles, i.e., articles prepared by subjecting glass articles to a controlled heat treatment to effect crystallization in situ, are well known to the art. The method for producing such articles customarily involves three fundamental steps: first, a glass-forming batch is melted; second, the melt is simultaneously cooled to a temperature at least below the transformation range thereof and a glass body of a desired geometry shaped therefrom; and, third, the glass body is heated to temperatures above the transformation range of the glass in a controlled manner to generate crystals in situ, this heat treatment frequently being termed "ceramming".
Frequently, the glass body is exposed to a two-stage heat treatment. In this treatment, the glass will be heated initially to a temperature within, or somewhat above, the transformation range for a period of time sufficient to cause the development of nuclei in the glass. Thereafter, the temperature will be raised to levels approaching, or even exceeding, the softening point of the glass to cause the growth of crystals on the previously-formed nuclei. The resultant crystals are commonly more uniformly fine-grained, and the articles are typically more highly crystalline.
Because glass-ceramic articles are generally highly crystalline, viz., greater than 50% by volume crystalline, they are normally mechanically stronger than the precursor glass articles from which they were derived. Hence, annealed glass bodies conventionally demonstrate modulus of rupture values in the range of about 5,000-10,000 psi, whereas the glass-ceramic product will exhibit moduli of rupture over the interval of 10,000-20,000 psi. Although the latter values represent a significant improvement, numerous investigations have been undertaken to enhance the mechanical strength of glass-ceramic bodies.
Major research efforts in strengthening glass-ceramic bodies have, however, been concentrated in the area of developing compressive stresses within a surface layer on a body. Two such methods of enhancing strength have found commercial application. One has involved applying, or forming, a surface layer of different chemical or crystalline composition, e.g., a glaze, having a coefficient of thermal expansion lower than that of the body. A second has comprehended subjecting the body to chemical strengthening via an ion exchange reaction. Both of those techniques are effective in increasing the mechanical strength of glass-ceramic articles, but both also have practical disadvantages.
Compression strengthening requires the body to be subjected to a further process which adds cost to the product. More importantly, however, the procedure does not enhance the toughness of an article. Toughness imparts resistance to catastrophic failure even when impact damage does occur. Absent such resistance, internal tension causes a body to fragment explosively into a larger number of small pieces. This phenomenon is especially significant when the product is designed for consumer goods where it is desired that any breakage be of a "gentle" nature with a resultant few large pieces.
In contrast, efforts to develop glass-ceramics having improved intrinsic, that is "body", strength have been modest. Also, little attention has been given to developing inherently tough materials by controlling the crystallization pattern or structure.
The present invention evolved out of a continuing search for tough glass-ceramic materials having high intrinsic strength. The benefits of such an inherently strong body, as compared to a compression strengthened body, are apparent. Violent or explosive breakage, associated with the large stress concentrations in compression strengthened ware, is avoided. The body is relatively insensitive to surface flaws, such as surface crazing or impact bruises. The tendency of minor surface cracks to grow under stress, the source of delayed breakage, is avoided.
It had been observed that glass-ceramics based on two-dimensional or platy crystals appeared to be stronger than those based on framework or three-dimensional crystals, other factors, such as grain size and percent crystallinity, being equal. It followed then that a one-dimensional crystal, such as the chain-silicates, might be a fertile area to explore if developed with considerable anisotropy as acicular forms.
An initial consideration was ability to develop a glass from which the crystal might form. Unfortunately, most compositions suggested by the desired chain silicate crystal forms failed to melt, formed very fluid glasses, were otherwise difficult to handle, devitrified uncontrollably, or resisted internal nucleation.
One family that showed promise, the canasite family, is disclosed in a copending application, Ser. No. 308,143 filed Oct. 5, 1981 by G. H. Beall, now U.S. Pat. No. 4,386,162. Also disclosed are related synthetic crystal forms, agrellite and fedorite. The canasite crystal structure is there described as a multiple chain silicate exhibiting an anisotropic, blade-like crystal habit. Structurally, the crystals are composed of parallel silicate chains crosslinked to make a long, box-like backbone in which the potassium ions rest. These complex chain units are crosslinked into groups of four, and are separated by networks composed primarily of Na(O,F).sub.6 and Ca(O,F).sub.6 octahedra. Some articles wherein canasite comprises essentially the sole crystal phase have displayed moduli of rupture in excess of 50,000 psi. The interlocking, blade-like morphology of the crystals is assumed to account for the high strength of the final product.
Agrellite and fedorite are also anisotropic silicates. Little is known of the structure of fedorite, although it evidences some similarity to the micaceous silicates. Agrellite is a tubular chain silicate of acicular habit. Because these crystals do not demonstrate as extensive an interlocking morphology as canasite, the mechanical strength of articles, wherein essentially the sole crystal phase is agrellite and/or fedorite, does not normally exceed about 25,000 psi.
The canasite glass-ceramics have a number of desirable features with respect to molded products such as dinnerware. In particular, they exhibit an intrinsic strength and toughness that reduces the possibility of either catastrophic or explosive breakage.
The canasite glass-ceramics are handicapped, however, by a general tendency to sag and deform during the ceramming cycle. This necessitates use of special supports and controlled firing conditions to avoid deformed ware. Furthermore, the high thermal coefficient of expansion, as well as the need for formers, militate against combining glazing and ceramming, a combination that can be most important cost-wise.